In the recovery of oil from a reservoir rock, generally only a fraction of the originally present oil is successfully extracted by primary recovery processes. In this case the oil reaches the surface as a result of the natural reservoir pressure. In secondary oil recovery, water is usually injected into one or more injection wells of the formation and the oil is driven to one or more production wells and then brought to the surface. This so-called water flooding as a secondary measure is relatively inexpensive and consequently is often used but in many cases leads to only a slight additional removal of oil from the reservoir.
An effective displacement of the oil, which is more expensive but in view of the future oil shortage is economically necessary, can be accomplished by tertiary measures. This includes processes in which either the viscosity of the oil is lowered and/or the viscosity of the reflooding water is increased and/or the interfacial tension between water and oil is lowered.
Most of these processes can be classified either as solution or mixture flooding, thermal oil recovery processes, surfactant or polymer flooding and as combinations of several such processes.
Thermal recovery processes comprise the injection of steam or hot water or they occur as subsurface combustion. Solution or mixture processes consist in injection of a solvent for the oil into the reservoir, and this can be a gas and/or a liquid.
In surfactant processes distinctions are made, depending on the surfactant concentration, and optionally the surfactant type and additives: these are surfactant-supported water flooding (a process which can serve, e.g., for increasing the injectivity of injection wells or represent a "low-tension process"), micellar flooding and emulsion flooding. The action of the surfactant flooding is based in the first place on a marked lowering of the interfacial tension between oil and flood water. In addition, the wettability of the rock surface and mobility ratio are very important. Favorable mobility ratios between oil and water are achieved by polymers.